In a nuclear reactor, moderated by means of light water, the fuel exists in the form of fuel rods, each of which contains a stack of pellets of a nuclear fuel arranged in a cladding tube. A fuel bundle comprises a plurality of fuel rods arranged in parallel with each other in a certain definite, normally symmetrical pattern, a so-called lattice. The fuel rods are retained at the top by a top tie plate and at the bottom by a bottom tie plate. To keep the fuel rods at a distance from each other and prevent them from bending or vibrating when the reactor is in operation, a plurality of spacers are distributed along the fuel bundle in the longitudinal direction. A fuel assembly comprises one or more fuel bundles, each one extending along the main part of the length of the fuel assembly. Together with a plurality of other fuel assemblies, the fuel assembly is arranged vertically in a reactor core. The core is immersed into water which serves both as coolant and as neutron moderator.
Since the coolant in a boiling water reactor is boiling, a ratio between water and steam is formed which varies axially in the core. At the bottom of the core, the temperature of the coolant is lower than the boiling temperature and is thus in a single-phase state, that is only water. At the top of the core, where the coolant has reached the boiling temperature, part of the water is transformed into steam, and the coolant is thus in a two-phase state. The higher up in the core, the greater is the percentage of steam in relation to the percentage of water. In the uppermost part of the core, the fuel rods are only covered with a thin film of water, outside of which steam mixed with water droplets is flowing, so-called annular flow.
If the thermal flow from a fuel rod becomes very great in relation to the coolant flow, there may be a risk of dryout. Dryout means that the liquid film becomes so thin that it is not capable of holding together, but it breaks up and forms dry wall portions, which locally leads to a considerably deteriorated heat transfer between the fuel rod and the coolant water resulting in a greatly increased wall temperature of the fuel rod. The increased wall temperature may lead to damage with serious consequences arising on the fuel rods. The risk of dryout exists substantially in the upper part of the fuel assembly.
Because of its lower density, steam is much inferior to water as moderator, which during operation of the reactor means that the higher up in the fuel assembly, the worse the moderation. In the core the fuel assemblies are surrounded by water which gives a good moderation of fuel rods near the fuel channel. In fuel rods in the central parts of the fuel assembly, on the other hand, inferior moderation will occur. Above all the central parts of the upper part of the fuel assembly will have an insufficient moderation. The reactivity of the reactor depends on the ratio of uranium to moderator. To obtain an optimum uranium-to-moderator ratio, the quantity of uranium should be smaller and the lattice space, that is, the free space between the fuel rods, should be larger in the upper part of the fuel assembly than in the lower part thereof.
Factors which are important to take into consideration when optimizing the fuel assembly are, in addition to reactivity and dryout, limitation of the linear load of the fuel rods, shutdown margin, and pressure drop.
A constantly recurring problem with boiling water reactors is how best to optimize the fuel assembly both axially and laterally with respect to uranium quantity and lattice space. Laterally, an optimization may be made, for example, by the choice of the diameter of the fuel rods, the distances between the fuel rods, and the number of fuel rods. A well-known method of achieving an axial optimization is to replace some of the fuel rods by part-length fuel rods. Part-length fuel rods have a shorter axial length than the traditional full-length fuel rods. Another method of achieving an optimization of the uranium quantity both axially and laterally is to vary the enrichment of the fuel in the fuel rods, which is shown in the German patent DE 40 14 861 A1. This patent shows a fuel assembly which has fuel rods with different enrichment in different lattice positions and certain of the fuel rods have several different enrichment contents axially.
A disadvantage with the above-mentioned optimization methods is that they are not capable of separately providing sufficiently efficient optimization of fuel and lattice space. With a conventional fuel assembly, it is difficult to achieve a good optimization in a simple manner. A solution to this problem is shown in International patent document PCT/SE95/01478 (Publ. No. WO 96/20483) which shows a flexible fuel assembly which can be optimized in a simple manner, both axially and laterally. The flexible fuel assembly comprises a plurality of fuel units stacked on top of each other, each comprising a plurality of fuel rods extending between a top tie plate and a bottom tie plate. The fuel units are surrounded by a common fuel channel with a substantially square cross section.
The needs of axial and lateral optimization differ between various reactors and various operating conditions. It is, therefore, desirable to be able to offer, for each individual customer, a fuel assembly which is optimized for the special needs of each individual customer. One problem is that it may be very expensive to supply different fuel assemblies to different customers since it requires a large number of different components which are both to be manufactured and be kept in stock.